This invention relates to improved tellurium compositions and a method for applying tellurium coatings to metallic surfaces. These coating compositions are characterized by the presence of tellurium and an organic acid solubilizing agent.
As used herein, the term "coating" refers to a material bonded to the surface of a metal which differs chemically from the metal itself. A particular example of a coating is a phosphate-based conversion coating. Such a coating is formed by chemical interaction between a phosphate-containing coating composition and the metal substrate being treated.
Conversion coatings are used to enhance the corrosion resistance of treated metal surfaces and to improve the adherence of paints and other coatings to these surfaces. As practiced in the art, conversion coatings are generally applied to metallic surfaces as iron phosphate, zinc phosphate or manganese phosphate. For example, a conversion coating may be produced by contacting a metal surface with a composition comprising a phosphate source, an acid and an accelerator. Typical accelerators used for this purpose include molybdenum, vanadium, nickel and tungsten salts.
Prior to application of a conversion coating, the metallic surface to be treated is generally cleaned to remove oil, grease, and other impurities. These impurities may act as mechanical barriers to conversion coating compositions or solutions, and can either interfere with or completely prevent adherence of the conversion coating to the metallic surface being treated.
After cleaning, the metallic surface is typically contacted with a conversion coating solution which comprises an acid, a phosphate source, an oxidizer and an accelerator. The surface is then generally rinsed with water to remove unreacted reagents and phosphate salts. Finally, a chromate, nitrate, or acid sealing rinse may be applied to the surface being treated, prior to painting.
Several disadvantages inherently plague conventional conversion coating methods, such as iron phosphate coating methods. Key among these is that iron phosphate processes generally produce coatings which provide less corrosion resistance than zinc phosphate coating processes. Since zinc phosphate processes are generally more complex and more costly to utilize, and are environmentally undesirable, there is a long-felt need in the art for a convenient, inexpensive method of providing corrosion-resistant conversion coatings. This need is met by the instant method and compositions.
It is therefore an object of this invention to provide improved tellurium compositions and an improved method for applying a uniform, durable tellurium coating to a metallic surface which provides corrosion resistance to the substrate being treated. This object is accomplished by utilizing tellurium coating compositions which contain a tellurium ion source and a solubilizing agent selected from the group consisting of .alpha.-substituted organic acids to form a coating characterized by the presence of tellurium. Particular examples of the instant solubilizing agents include, but are not limited to, hydroxyacetic acid, tannic acid, tartaric acid, citric acid, 2,6-pyridine-dicarboxylic acid, lactic acid, glucono .delta.-lactone (gluconic acid), 2-puroic acid, thiophene-2-carboxylic acid, 2,3-pyridine dicarboxylic acid, phosphonoacetic acid, thiophene-2-acetic acid and mercaptoacetic acid.
Any metallic surface can be treated according to the instant invention, including but not limited to galvanized surfaces, stainless steel surfaces, mild steel surfaces and aluminum surfaces.
This and other objects of the instant invention are accomplished by use of the instant compositions and the method disclosed herein. The instant coating compositions and method allow the application of uniform tellurium coatings to metallic surfaces, particularly in the mid-pH range. The method can be utilized at any temperature up to boiling, and the resulting coating provides corrosion resistance to the substrate. The instant coatings also generally improve the appearance of paints and other coatings subsequently applied to treated metallic surfaces.
The MERCK INDEX, Tenth Edition, discloses that tellurium is a reagent which produces a black finish on silverware.
U.S. Pat. No. 4,713,121 discloses phosphate conversion coatings which contain first and second divalent metal elements, such as cobalt and zinc.
U.S. Pat. No. 4,391,855 discloses a coating method which utilizes compositions containing a powdered metal dispersed in a bonding material as a corrosion inhibitor.
U.S. Pat. No. 4,149,909 discloses the use of chlorates and bromates as accelerators and hydroxylamine sulfate as a reducing agent in phosphatizing compositions used to produce iron phosphate coatings.
U.S. Pat. No. 4,595,424 discloses phosphate coating solutions for use on zinc surfaces which contain a phosphate ion source, a zinc and/or manganese ion source and a complex of fluoride ions.
U.S. Pat. No. 4,634,295 discloses a method for improving corrosion resistance of metal substrates which requires application of a direct current to a previously zinc-phosphated metal surface in an acidic solution containing zinc, phosphate and chloride ions.
Copending application U.S. application Ser. No. 361,087 discloses tellurium compositions and methods for applying the same to metallic surfaces. Copending application U.S. Ser. No. 739,010 discloses the use of .alpha.-substituted organic acids to solubilize tellurium.